Paralleling voltage regulators

ABSTRACT

Circuits and methods for paralleling voltage regulators are provided. Improved current sharing and regulation characteristics are obtained by coupling control terminals of the voltage regulators together which results in precise output voltages and proportional current production. Distributing current generation among multiple paralleled voltage regulators improves heat dissipation and thereby reduces the likelihood that the current produced by the voltage regulators will be temperature limited.

BACKGROUND OF THE INVENTION

The invention relates to voltage regulators. More particularly, theinventions described herein relate to systems and methods forinterconnecting voltage regulators to provide simple current sharingtechniques and improved regulation characteristics.

Voltage regulators are found in virtually every device that requireselectricity. The purpose of a voltage regulator circuit is to controland regulate voltage from a power source to a load, typically throughcertain conditioning and regulation circuitry. A typical application ofvoltage regulator circuitry is to convert AC power, provided by a powerutility, to a regulated DC voltage suitable for use with consumerelectronics. Such power supplies are controlled by a voltage regulator.Although voltage regulators are implemented as stand alone systems,often they are constructed as integrated circuits (ICs) and used invarious applications including communication and computing systems.

Two or more voltage regulators may be connected together to providegreater output current. Factors favoring the parallel of connectionvoltage regulators include the need to dissipate heat over a wider areaas well as increase output current. In some instances, many voltageregulators may be connected together to provide additional voltage to aload. In other instances, the voltage regulators may be connected inparallel to provide additional load current. In such instances, theconnected voltage regulators are typically configured in an attempt toshare current to the load. This may be done in order to promote loadbalancing and/or to maintain system operation within a desired peaktemperature range.

As manufactured, voltage regulators experience wide variation in outputvoltage, thereby making current matching between directly paralleledvoltage regulators relatively difficult to achieve.

The portion of load current supplied by each parallel connected voltageregulator is often dependent on the difference in output voltage andoutput impedance of the respective voltage regulators. Thus, whenvoltage regulators are connected in parallel, the regulator having thehigher output voltage typically supplies more current than the supplywith a lower output voltage. As a result, the supply with the higheroutput voltage may provide most or all of the current to the load.Moreover, the regulator providing the highest output voltage may limitin overload before the other regulators begin to supply current. Thisunbalanced condition is further exacerbated if the regulator with thehigher output voltage also has the lower output impedance of the two (ormore) supplies.

The unequal sharing of load current by paralleled voltage regulators maydegrade both the performance and reliability of a power system. Thisproblem is of particular concern for surface mounted voltage regulatorsdue to the inherent limitation of power dissipation when mounted on acircuit board. In certain situations, the additional thermal stressresulting from such severe current imbalances may reduce componentlifetime in the sourcing supply by 50% or more.

Various techniques have been used to balance current among parallelconnected voltage regulators. One known current sharing techniqueinvolves the use of a “share bus” configuration in which output currentinformation is shared among the parallel connected regulators toregulate current. In such systems, a current sense resistor is used todevelop a voltage which represents the output current of the parallelconnected voltage regulators. This voltage is reproduced on the sharebus and monitored by the voltage regulators to determine how muchcurrent to provide. Because the power supplies are providing currentbased on both an internal error voltage (for individual supplyregulation) and the voltage on the share bus (for group regulation),current is supplied approximately equally. One drawback with thisarrangement, however, is the need for complicated controller circuitryand specialized interconnections among the voltage regulators.

Accordingly, it would be desirable to provide circuits and methods forthe efficient sharing of current among paralleled voltage regulatorsthat does not degrade voltage regulation.

Moreover, it would be desirable to provide circuits and methods for heatdissipation in parallel coupled voltage regulators that result inimproved current sharing.

SUMMARY OF THE INVENTION

Circuits and methods for paralleling voltage regulators are providedwhich efficiently share current among paralleled voltage regulators andthat does not degrade voltage regulation. The parallel coupled voltageregulators of the present invention enjoy improved heat dissipation andcurrent sharing over a broad operating range.

In one embodiment, a method of coupling two or more voltage regulatorsin parallel to provide a combined output current is provided, includingproviding a first voltage regulator that generates a substantiallyconstant voltage, the first voltage regulator having a power outputstage; the first voltage regulator output stage having a control inputand an output, a second voltage regulator that generates a substantiallyconstant voltage, the second voltage regulator having a power outputstage; the second voltage regulator output stage having a control inputand an output, coupling the control input of the first voltage regulatoroutput stage to the control input of the second voltage regulator outputstage such that the voltage at an output of the first voltage regulatorand the second voltage regulator is substantially equal; and couplingthe output of the first voltage regulator to the output of the secondvoltage regulator in parallel such that current produced issubstantially equal to the sum of current produced by the first voltageregulator and the second voltage regulator.

In certain embodiments, the invention may further include minimizing thevoltage difference between the control input and the output of the firstregulator output stage. Ballast resistors having small resistance valuesmay be used in some embodiments to further improve the precision ofoutput current without sacrificing load regulation. Other aspects of theinvention include effective heat dissipation which minimizes hot spotsand the need for separately mounted voltage regulators and heat sinks insurface mounted circuit board applications.

In other embodiments, a voltage regulator suitable for implementation onan integrated circuit is provided that supplies a substantially constantoutput voltage and is suitable for coupling to one or more voltageregulators in parallel to provide a combined output current, the voltageregulator circuit comprising, a current reference circuit for providinga substantially constant output current, a set impedance coupled to thecurrent reference circuit for generating a substantially constant setvoltage from the substantially constant output current, an amplifiercircuit coupled to the current reference circuit and the set impedancethat generates a substantially constant output voltage based on the setvoltage, and a ballast impedance coupled to the output of the amplifiercircuit for establishing an output impedance of the voltage regulatorcircuit. In such embodiments, the load regulation of the voltageregulator may be substantially independent of output voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and advantages of the present invention willbe apparent upon consideration of the following detailed description,taken in conjunction with the accompanying drawings, in which likereference characters refer to like parts throughout, and in which:

FIG. 1 is a diagram of one embodiment of parallel connected voltageregulators in accordance with the principles of the present invention;and

FIG. 2 is a schematic diagram of another embodiment of parallelconnected voltage regulators constructed in accordance with theprinciples of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

A diagram of one embodiment of two parallel connected voltage regulatorsconstructed in accordance with the principles of present invention isshown in FIG. 1. As shown, system 100 generally includes voltageregulator circuits 110 and 120. Voltage regulator circuit 110 maygenerally include a voltage reference circuit 111 and a power outputstage driven by resistor 112 and operational amplifier 113. Similarly,voltage regulator circuit 120 may generally include a voltage referencecircuit 121 and a power output stage driven by resistor 122 andoperational amplifier 123. For simplicity, only two voltage regulatorsare shown in FIG. 1. However, additional voltage regulators may be addedas shown, if desired. This may be done, for example, to further improvethe aggregate current sourcing capability of system 100.

In operation, voltage reference circuits 111 and 121 may provide apredetermined voltage through resistors 112 and 122 to the non-invertinginput of amplifiers 113 and 123 respectively, which are preferablyconfigured as voltage followers (i.e., provide a current gain with unityvoltage). This causes amplifiers 113 and 123 to generate an output withvoltages substantially equal to their input. Each output signal may bepassed through ballast resistors 114 and 124 to generate a compositeoutput V_(OUT), which is a combination of the two outputs. The ballastresistors help establish a maximum current imbalance between voltageregulators 110 and 120 at full output and help minimize overall currentimbalance of system 100.

As shown in FIG. 1, because the non-inverting inputs of amplifiers 113and 123 are coupled together, their respective outputs are substantiallyequal in voltage. Each non-inverting input may be considered a controlinput of regulators 110 and 120. If ballast resistors 114 and 124 areconfigured to have substantially the same value, then the-output currentof each regulator is substantially the same as well. Thus, for example,if voltage regulators 110 and 120 are each configured to provide a 5volt output at 1 amp (5 watts), the total output of system 100 would be5 volts (same voltage) but at 2 amps (10 watts).

In some embodiments, an external voltage regulator circuit (not shown)may be coupled to the control input of regulator 110 and/or 120 whichmay be used to establish the output voltage of the paralleledregulators. This allows regulators 110 and 120 to be programmed bysources other than reference circuits 111 and 121. In this case, thevoltage follower circuits in each regulator continue to provide theoutput voltage and shared current as described herein, but based on thevalue established by the external source. In such embodiments, referencecircuits 110 and/or 120 may be turned OFF or disconnected from thevoltage follower circuits. Moreover, voltage references 111 and/or 121may be programmable, so that a manufacturer or end user and set thedesired output voltage of the paralleled regulators.

Small mismatches in the output voltages are impressed across the ballastresistors. If the voltage followers are precise, with low offset betweeninput and output, ballast resistors with very low impedance can be used.In general, the lower the value of the ballast resistor, the lessdegradation in load regulation. Such ballast resistors can be made of ashort piece of printed circuit board trace. Generally speaking, thevalue of the ballast resistors is a function of the precision of thevoltage follower circuits. The greater the precision of the voltagefollowers, the lower the value of the ballast resistors.

In the example above, each voltage regulator provides substantially thesame current to a load connected to V_(OUT) (not shown) and thusprovides an effective current sharing architecture. Another benefit ofthis general configuration is that it uses commonly available componentsand eliminates the need to generate system based signals for outputcurrent regulation, greatly simplifying the regulation circuitry.

For example, in integrated circuits, very accurate implementations ofthe voltage follower circuitry described above may be obtained,allowing, in some instances, the potential difference between amplifiers113 and 123 to be as low as one to two millivolts of their input. Insome embodiments, the offset associated with the voltage followers maybe minimized by trimming components during the manufacturing process.

Because the output voltages of each regulator are substantially equal,the value of ballast resistors 114 and 124 may be very low, desirablyreducing or eliminating any load regulation loss associated with theresistive ballasting such that the performance of the supplies remainsubstantially unaffected. In some embodiments, load regulation loss maybe in 5-10 millivolt range, which is well within the 1% load regulationrequirement commonly specified for voltage regulators. Such low valueballasting resistors may be obtained from less than an inch of copper PCboard used to connect the supplies and may have a resistance in theorder of about 10 milliohms. Other suitable resistances may be specifiedby a manufacturer of specific embodiments of the devices describedherein.

In addition to providing increased output current, the present inventionalso provides a means for dissipating heat over a larger area duringcircuit operation. For example, when a voltage regulator such asregulator 110 is surface mounted on a circuit board, the amount of heatthat can be dissipated by that regulator is limited due to variousphysical constraints, which in turn limits the maximum output current ofthe regulator. By connecting regulators 110 and 120 in parallel, theheat generated is spread out across a wider area, providing betterdissipation and thus better cooling, which allows the two regulators toprovide their rated current without running into their thermal limit.

Furthermore, this reduces the number and intensity of “hot spots” on acircuit board, lowers overall peak temperatures and reduces the need forseparately mounted voltage regulators with large heat sinks. As aresult, multiple voltage regulators may be mounted on the same orclosely spaced circuit boards to achieve a desired output currentwithout restriction due to elevated operating temperatures.

Referring now to FIG. 2, another embodiment 200 constructed inaccordance with the principles of the present invention is shown.Circuit 200 is similar in many respects to the circuit described in FIG.1 and generally includes components and functional blocks which havebeen numbered similarly to denote similar functionality and generalcorrespondence. For example, circuit 200 includes voltage regulatorcircuits 210 and 220 (voltage regulators 110 and 120 in FIG. 1),amplifier circuits 213 and 223 (amplifier circuits 113 and 123respectively in FIG. 1), and ballast resistors 214 and 224 (ballastresistors 114 and 124 in FIG. 1).

As shown, system 200 may operate in substantially the same way as system100, with the exception of reference circuits 211 and 221. Rather thanoperate as voltage-based circuits as described in FIG. 1, referencecircuits 211 and 221 are configured to provide a substantially constantreference current, with the output voltage of each regulator being setby a resistor to ground. As shown, set resistors 212 and 222 establish avoltage V_(SET) which is provided to the non-inverting input ofamplifiers 213 and 223. Once this set voltage is established, circuit200 may operate similarly to circuit 100 described above. One benefit ofusing current references is voltage dividers are not required, whichprovides improved load regulation. Moreover, in such embodiments, loadregulation is independent of output voltage. Furthermore, ballastresistors do not need to be scaled to output voltages.

As in circuit 100, the value of ballast resistors 214 and 224 may bevery low, and achieve the same load regulation benefits described inconnection with the circuit of FIG. 1. Moreover, voltage regulators 210and 220 with different current sourcing capabilities may be coupled inparallel as shown with ballast resistances 214 and 224 scaled betweenthe voltage regulators to ensure current sharing in the ratio ofavailable current. For example, if the current sourcing capability ofvoltage regulator 210 is five times greater than that of supply 220,ballast resistors 224 and 214 may be configured in a five to one ratioto allow current to be drawn proportionately from each supply and ensurethat system 200 provides a maximum output current.

In some embodiments, resistors 212 and 222 may be external andadjustable to set the output voltage of circuit 200. In otherembodiments, only one resistor may be used to set the voltage value formultiple regulators (not shown). Such a resistor may be internal orexternal and fixed or adjustable. In the case where multiple setresistors are used for multiple supplies, the value of the set resistorsmay need to be selected in view of the resulting parallel combination inorder to achieve the desired resistance and thus the desired outputvoltage.

In certain other embodiments, e.g., such as those used for integratedcircuits, it may be desirable to employ voltage follower circuits thathave a negative temperature coefficient (i.e., a voltage followercircuit whose output voltage decreases after the temperature exceeds acertain point or decreases with temperature). In such embodiments, thenegative temperature coefficient itself may be used as a ballastingmechanism (e.g., with or without the use of ballasting resistors).

For example, in operation, assume regulators 210 and 220 are configuredsuch that they both have substantially the same negative temperaturecoefficient. If one of these supplies begins to source more current thanthe other, or provides current above that specified in a predeterminedratio, that supply will begin to rise in temperature. The built intemperature coefficient of that regulator will provide temperatureregulation, which reacts to the temperature increase (i.e., unequaltemperature rise) and causes its output voltage to correspondinglydecrease. This, in turn, causes the current to adjust as well (based onthe thermal resistance characteristics and temperature of theregulators). As a result, the output current of the supplies return to astate where the output current is balanced to a certain degree (e.g.,based on how closely certain factors are matched such as temperaturecoefficient, heat sinking capability, ambient temperature, etc.).

In such embodiments, current sharing based on temperature regulationdoes not require the use of ballast resistors (e.g., resistors 114, 124,214 and 224). However, small impedances may be used if desired toachieve further output precision. Moreover, in some embodiments, heatsinking by itself may be used as a means of establishing current ratiosbetween supplies. For example, if two (or more) supplies capable ofproducing substantially the same or similar current, with substantiallythe same negative temperature coefficient are provided with differentheat sinks, the supply with the lesser heat sinking capability mayprovide proportionally less current based on its temperature limits.Thus, if an operating temperature range is known, various paralleledvoltage regulators can be provided with heat sinks that will allow themto provide current in a desired ratio based on their respective heatdissipation characteristics.

Similarly, in some embodiments, the temperature regulation itself may beused as a means of establishing current ratios between supplies. Forexample, if two (or more) regulators capable of producing substantiallythe same or similar current are provided with different negativetemperature coefficients, the regulator with the more negativetemperature coefficient may provide proportionally less current basedits temperature limits.

Moreover, it will be apparent from the foregoing that both heat sinkingand temperature coefficient factors may be combined to establish currentsharing parameters between voltage regulators, e.g., supplies with lessnegative temperature coefficients having more heat sinking capabilitymay be coupled with supplies having more negative temperaturecoefficients and less heat sinking capability, the former providingproportionally more current than the latter, etc. Such implementationsmay optionally include ballast resistors, if desired to further improveprecision. Other configurations are possible as well.

It will be understood that the systems and methods described herein havebroad based applicability and may be employed in multiple differentcontexts. For example, the systems described above may be used in “box”type power supplies such as those commercially produced by LambdaCorporation, Kerco, or Agilent, or in integrated circuit type voltageregulators such as those produced by Linear Technology Corporation ofMilpitas Calif., the assignee of this patent application. Accordingly,commonly available circuitry referred to above may include externalcircuitry commonly available in a box implementation such as easilyadded components on a circuit board, or, in an IC implementation, mayinclude amplifiers or such components which may be added during designat little or no additional expense.

Moreover, voltage reference circuits 111 and 121 may include anycircuitry suitable for supplying a substantially constant predeterminedvoltage through a resistor, and may include any suitable configurationof switching or linear based regulator or other conventional referencecircuitry. Similarly, current reference circuits 211 and 212 may includeany circuitry suitable for supplying a substantially constantpredetermined current, and may include any suitable configuration ofswitching or linear based regulator or other conventional referencecircuitry. Such reference circuits may include the reference circuitsdescribed in co-pending U.S. patent application entitled Bandgap Voltageand Current Reference Ser. No. 11/731,279 filed Mar. 30, 2007 assignedto the assignee of this patent application, which is hereby incorporatedby reference in its entirety.

Further, although ballast resistors 114, 124, 214 and 224 are shown asexternal to voltage regulators 110, 120, 210 and 220 such resistors may,in some embodiments, be internally based. In some embodiments, bondingwires commonly found in an integrated circuit package can also be usedas ballast. Also, the regulation itself may act as a ballast.Furthermore, the voltage follower circuitry described herein may beconstructed using any suitable topology known in the art. Although suchcircuits are described herein with a unity voltage gain it will beunderstood that gains other than unity may be used if desired. At gainsabove unity, less precise matching is typical and accurate sharing ismore difficult. Certain older voltage regulators may operate with a 1-3%error in their output, an may operate above unity gain. Such regulatorshave difficultly being coupled in parallel as described herein and mayrely on large ballast resistors which degrade load regulation. Moreover,in some embodiments, where amplifiers are running at or near their poweror ground rails, offset voltages may be employed if desired to preventor insure cutoff.

Although preferred embodiments of the present invention have beendisclosed with various circuits connected to other circuits, personsskilled in the art will appreciate that it may not be necessary for suchconnections to be direct and additional circuits may be interconnectedbetween the shown connected circuits without departing from the spiritof the invention as shown. Persons skilled in the art also willappreciate that the present invention can be practiced by other than thespecifically described embodiments. The described embodiments arepresented for purposes of illustration and not of limitation, and thepresent invention is limited only by the claims which follow.

1. A method of coupling two or more voltage regulators in parallel toprovide a combined output current, the method comprising: providing afirst voltage regulator that generates a substantially constant voltage,the first voltage regulator having a power output stage; the poweroutput stage having a control input and an output; providing a secondvoltage regulator that generates a substantially constant voltage, thesecond voltage regulator having a power output stage; the power outputstage having a control input and an output; coupling the control inputof the first voltage regulator output stage to the control input of thesecond voltage regulator output stage such that the voltage at an outputof the first voltage regulator and an output of the second voltageregulator is substantially equal; coupling the output of the firstvoltage regulator to the output of the second voltage regulator inparallel such that the combined output current produced is substantiallyequal to the sum of current produced by the first voltage regulator andthe second voltage regulator; and configuring the first voltageregulator output stage to include a voltage follower amplifier coupledto a first voltage reference circuit such that the output voltage of thefirst voltage regulator is substantially equal to an output voltage ofthe first voltage reference circuit.
 2. The method of claim 1 furthercomprising minimizing the voltage difference between the control inputof the first voltage regulator output stage and the output of the firstvoltage regulator output stage.
 3. The method of claim 2 wherein theoutput of the first voltage regulator output stage is adjusted tosubstantially match the voltage on the control input of the firstvoltage regulator output stage.
 4. The method of claim 1 furthercomprising minimizing the voltage difference between the control inputof the second voltage regulator and the output of the second voltageregulator output stage.
 5. The method of claim 4 wherein the output ofthe second voltage regulator output stage is adjusted to substantiallymatch a voltage on the control input of the second voltage regulator. 6.The method of claim 1 further comprising configuring the second voltageregulator output stage to include a voltage follower amplifier coupledto a voltage reference circuit such that the output voltage of thesecond voltage regulator is substantially equal to an output voltage ofthe second voltage reference circuit.
 7. The method of claim 1 furthercomprising configuring the first voltage regulator output stage toinclude a voltage follower amplifier coupled to a first currentreference circuit such that the output voltage of the first voltageregulator is substantially equal to an output voltage generated by thefirst current reference circuit and a first impedance.
 8. The method ofclaim 7 wherein the first impedance is adjustable.
 9. The method ofclaim 7 wherein the first impedance is external to the first voltageregulator.
 10. The method of claim 7 further comprising configuring thesecond voltage regulator output stage to include a voltage followeramplifier coupled to a second current reference circuit such that theoutput voltage of the second voltage regulator is substantially equal toan output voltage generated by the second current reference circuit anda reference impedance.
 11. The method of claim 10 wherein the referenceimpedance is adjustable.
 12. The method of claim 10 wherein thereference impedance is the first impedance.
 13. The method of claim 1further including providing a first ballast impedance coupled to theoutput of the first output stage, such that a first voltage drop isestablished by the current of the first voltage regulator.
 14. Themethod of claim 11 wherein the first ballast impedance is formed, atleast in part, by bondwire in an integrated circuit package.
 15. Themethod of claim 13 further including providing a second ballastimpedance coupled to the output of the second output stage, such that asecond voltage drop is established by the current of the second voltageregulator.
 16. The method of claim 15 wherein the first ballastimpedance is formed, at least in part, by bondwire in an integratedcircuit package.
 17. The method of claim 1 further comprising disposingthe first voltage regulator and the second voltage regulator indifferent devices.
 18. The method of claim 1 further comprisingproviding the first voltage regulator with a first negative temperaturecoefficient and the second voltage regulator with a second negativetemperature coefficient.
 19. The method of claim 18 wherein the firstand second negative temperature coefficients are substantially equal.20. The method of claim 18 wherein the first and second negativetemperature coefficients are substantially different from one anotherand the current provided by the first and second voltage regulators isproportional to a ratio of the first and second negative temperaturecoefficients.
 21. The method of claim 1 further comprising providing thefirst voltage regulator with a first heat dissipation capability and thesecond voltage regulator with a second heat dissipation capability. 22.The method of claim 21 wherein the first and second heat dissipationcapabilities are substantially equal.
 23. The method of claim 20 whereinthe first and second heat dissipation capabilities are substantiallydifferent and the current provided by the first and second voltageregulators is proportional to a ratio of the first and second heatdissipation capabilities.
 24. A device that provides a substantiallyconstant output voltage suitable for coupling to one or more devices inparallel to provide a combined output current, the device comprising: afirst voltage regulator that generates a substantially constant voltage,the first voltage regulator having a power output stage; the firstvoltage regulator power output stage having a control input and anoutput; wherein the control input of the first voltage regulator poweroutput stage is configured to be coupled to a second voltage regulator,which causes a voltage at the output of the first voltage regulator tobe substantially equal to a voltage at an output of the second voltageregulator; and which causes an output current produced by the firstvoltage regulator through a first ballast impedance to be proportionalto an output current produced by the second voltage regulator through asecond ballast impedance.
 25. The device of claim 24 wherein the firstvoltage regulator includes a first current reference circuit thatproduces a first current and the second voltage regulator includes asecond current reference circuit that produces a second current suchthat the output voltage of the device is substantially equal to a sum ofthe first current and the second current multiplied by a set impedance.26. The device of claim 24 wherein the output current produced by thefirst voltage regulator is substantially equal to the output currentprovided by the second voltage regulator.
 27. The device of claim 24configured to minimize the voltage difference between the control inputof the first voltage regulator and the output of the first voltageregulator output stage.
 28. The device of claim 27 configured such thatthe output of the first voltage regulator output stage is adjusted tosubstantially match a voltage on the control input of the first voltageregulator.
 29. The device of claim 24 wherein the first voltageregulator output stage is configured to include a voltage followeramplifier coupled to a first voltage reference circuit such that theoutput voltage of the first voltage regulator is substantially equal toan output voltage of the first voltage reference circuit.
 30. The deviceof claim 29 wherein the first voltage reference circuit is programmable.31. The device of claim 24 wherein the first voltage regulator outputstage is configured to include a voltage follower amplifier coupled to afirst current reference circuit such that the output voltage of thefirst voltage regulator is substantially equal to an output voltagegenerated by the first current reference circuit and a first impedance.32. The device of claim 31 wherein the first impedance is adjustable.33. The device of claim 31 wherein the first impedance is external tothe first voltage regulator.
 34. The device of claim 24 wherein thefirst ballast impedance establishes a first current dependent voltagedrop for the first voltage regulator.
 35. The device of claim 31 whereinthe first ballast impedance is formed, at least in part, by bondwire inan integrated circuit package.
 36. The device of claim 24 wherein thefirst voltage regulator has a negative temperature coefficient and thesecond voltage regulator has a negative temperature coefficient and theoutput current provided by the first voltage regulator is proportionalto a ratio of a temperature of the first voltage regulator and atemperature of the second voltage regulator.
 37. The device of claim 24wherein the first voltage regulator further includes a first heat sinkwherein the current provided by the first voltage regulator isproportional to a ratio of a heat dissipation capability of first heatsink and a heat dissipation capability of the second voltage regulator.38. A voltage regulator disposed on an integrated circuit that providesa substantially constant output voltage suitable for coupling to one ormore voltage regulators in parallel to provide a combined outputcurrent, the voltage regulator circuit comprising: a current referencecircuit for providing a substantially constant output current; a setimpedance coupled to the current reference circuit for generating asubstantially constant set voltage from the substantially constantoutput current; an amplifier circuit coupled to the current referencecircuit and the set impedance that generates a substantially constantoutput voltage based on the set voltage, and; a ballast impedancecoupled to the output of the amplifier circuit for establishing anoutput impedance of the voltage regulator circuit.
 39. The voltageregulator of claim 38 further wherein the ballast impedance is minimizedto improve load regulation.
 40. The voltage regulator of claim 39wherein the ballast impedance is formed by bondwire in the integratedcircuit.
 41. The voltage regulator of claim 38 wherein the amplifiercircuit further includes a control input, the substantially constant setvoltage being coupled to the control input, and wherein the voltageregulator is configured to minimize the voltage difference between thecontrol input of the amplifier and the output of the voltage regulator.42. The voltage regulator of claim 41 configured such that the output ofthe amplifier circuit is adjusted to substantially match the voltage onthe control input.
 43. The voltage regulator of claim 38 wherein theamplifier circuit is configured to include a voltage follower amplifiercoupled to the current reference circuit such that the output voltage ofthe voltage regulator is substantially equal to the set voltage.
 44. Thevoltage regulator of claim 38 wherein the first voltage referencecircuit is programmable.
 45. The voltage regulator of claim 38 whereinthe set impedance is adjustable.
 46. The voltage regulator of claim 45wherein the set impedance is external to the integrated circuit.
 47. Thevoltage regulator of claim 38 wherein load regulation is substantiallyindependent of output voltage.